In an electro-optical document reading apparatus, light is directed onto a document and light quanta reflected from elemental areas (pixels) of the irradiated surface are detected by a light-sensitive sensor and translated into video signals. The sensor may comprise an array of sensor elements, usually solid-state sensor elements such as photodiodes or charge-coupled devices (CCDs).
The output signals of the sensor should be equal when reading uniformly irradiated pixels of the same grey scale value. In practice however, the signals are not equal under those conditions. There are various causes of such discrepancies. In some cases signalling errors occur because of distorting effects of the optical system. When light from a uniformly irradiated surface is projected in reduced scale by an optical focusing system onto an image sensor the light flux on the sensor is greater near the central optical axis than in the peripheral region, due to the so-called cosine-to-the-fourth shading effect of the optical system. Variations in the video signal levels can also occur as a result of imperfections in the sensitivity characteristic of the sensor itself. As is well known, the transducing properties of electro-optical convertors are liable to suffer from one or another kind of defect, depending on their type. Such defects cause inaccuracies in the transducing of the grey level values of incident light signals.
Anomalous variations in the light output from the scanning light source, or obstruction of the emitted light by dirt on the lamp(s) or on the focusing system are further possible causes of inaccuracies in the reader performance.
It has been proposed in this art to compensate for the cosine-to-the-fourth shading effect of the optical system by using a light source with an appropriately non-uniform light output distribution or by providing an appropriately dimensioned light shield in front of the focusing lens or between the lens and the document to be read (see U.S. Pat. No. 4 220 978 and G.B. patent No. A 2 064 809). The compensating effect achieved in these ways does not deal with the other causes of reading errors which have been mentioned.
An image reading apparatus having video signal correcting means which correct for variations in sensor response characteristics and for illumination faults, as well as for cosine-to-the-fourth shading effects is described in U.S. Pat. No. 4 228 426. The correcting means use several pulse generators for generating a plurality of timing signals and therefore involve very complex electronic circuitry.
A correcting system which in principle is potentially more satisfactory was disclosed at the IEEE International Solid-state Circuits Conference by Mitsuo Togashi et al of Matsushita Graphic Communication Systems. Inc. of Japan. This system, which corrects for sensor response defects and cosine-to-the-fourth shading, makes use of a single chip signal-processor. During a first processing step a reference original is read by a sensor and a correction value for the video value pertaining to each pixel is determined and stored. During a second step, the document is read and the output video signal is corrected by amplification of that video signal in each pixel with a corresponding correction factor that was determined in the first step.
The correction factors used in the foregoing system are n-bit binary code correction words. Correction factors as are mentioned in the article by Mitsuo Togashi can be determined by means of the successive approximation technique, known in the art.
According to this technique. each word is developed from an initial value to the required value representing the required correction factor for that pixel by a procedure comprising a sequence of checking cycles. This sequence of checking cycles is performed during the period of time that is available in between the shifting out of the sensors read out register of the video value to which the correction factor is pertaining and of the video value corresponding with the next pixel.
Each checking cycle includes the steps of amplifying the signal delivered by the sensor with an n-bit binary code word initially having its most significant bit set to one and the other bits set to zero to give a modified video response signal and comparing the modified signal with a predetermined reference signal level to give a comparison signal. Each correction word is changed bit by bit as required in dependence on the comparison signals generated in the approximation cycles, thereby each time changing the bit next to the corrected bit in order of decreasing significance into one to bring the word progressively into approximation with the required coded correction factor.
When applying the successive approximation procedure for the development of coded correction factors for the sensor output of an electro-optical reading apparatus, it is necessary to perform at least n x N checking cycles, where n is the number of bits in a correction word and N is the number of pixels to which the sensor is exposed during a reading period. By a reading period of the sensor is meant the period of time for which the sensor or any individual sensor element is continuously irradiated before the accumulated energy is released and shift out as a video signal. N is the number of pixels in a scanned line, in the case of line-wise scanning, and is the number of pixels in a scanned surface area in the case of matrix scanning.
If the correction factors for use in an output correcting method as described in the paper by Mitsuo et al. are to be determined by means of the successive approximation technique, either the shift out period of the sensors shift register is to be extended as long as is needed for the performance of the entire successive aproximation procedure of each correction word pertaining to a pixel in the read line (which solution slows down the operation speed of the scanning device), or alternatively very fast and expensive electronic circuitry is to be used.
There are applications in which the duration of said shift out period has to be kept very short. Fast scanning, for examples, obliges that signals pertaining to scanned data are shifted out rapidly so as to enable reading of new data. In such applications the prior art successive approximation procedure can only be applied when high speed electronic circuitry is used.